At What Point in My Workflow Should I Apply a Mirror Modifier?
Either undo (CTRL+Z), or File &gt; Revert, or load a backup file, or manually remove mirrored vertices:

Press Numpad 7 if your object was mirrored around X or Y, or e.g. Numpad 1 if it was mirrored around Z, then press SHIFT + Z for X-Ray mode and drag-select all vertices that have been mirrored - and press Del or X to remove them.